1. Field of the Invention
The present invention relates to a Schmitt trigger circuit and a power supply monitoring apparatus.
2. Description of the Related Art
Recent years have seen the spread of electric vehicles. In such circumstances, a configuration has been developed in which electric power is output from an electric vehicle battery to the outside so as to use the power for emergency.
In order to implement such a configuration, a power supply control apparatus for electric vehicles has been developed. Such a power supply control apparatus includes a mechanical relay that electrically connects an electric vehicle battery for driving a vehicle to an outlet for outputting power.
Such a power supply control apparatus has a voltage output function and a current output function so as to report a voltage or a supply current of the vehicle battery to an external electronic control unit (ECU) while the power is supplied through the mechanical relay. Conventional technologies are described in Japanese Laid-open Patent Publication No 2014-033533 for example.
A vehicle battery for driving a vehicle is set for a relatively high voltage (e.g., 275 V to 650 V), and thus needs to be electrically insulated from an output circuit (a communication interface) that operates at a relatively low voltage (e.g., 12 V) and communicates with an external ECU.
For this reason, a relatively high-voltage circuit and a relatively low-voltage circuit are electrically insulated from each other using a photocoupler.
In addition, a voltage fluctuation range of a vehicle battery may be large, fluctuating from 40 V to 650 V for example. Furthermore, in order to ensure stable operation of a power supply control apparatus, a light emitting diode (LED) included in a photocoupler is driven by using a Schmitt trigger circuit having hysteresis so that monitoring results do not vary with some voltage fluctuation in a certain voltage range of a vehicle battery that can supply power for emergency.
Meanwhile, a current proportional to a detected voltage flows through an LED included in a photocoupler, and an external ECU simply determines whether a voltage equal to or larger than a certain voltage is detected. In consideration of the life of the LED or other conditions, it is desired to limit current when a voltage of the vehicle battery is equal to or larger than the certain voltage. Improvement in the accuracy of voltage measurement has also been desired.